1. Field of the Invention
This invention relates to a stern tube bearing system of contra-rotating propeller for large-size marine vessels.
2. Description of the Prior Art
FIGS. 14 through 18 show conventional stern tube bearing systems of contra-rotating propeller.
Referring to these drawings, marine vessels employing conventional contra-rotating propeller systems use the well-known vessel-propulsion method in which an inner shaft 02 having at its point a rear propeller 01 and an outer shaft 04 having at its point a front propeller 03 are rotated in opposite directions via a reversing device by a prime mover not shown (such as diesel engine, steam turbine, or gas turbine). To drive propeller shafts smoothly, generally, a front bearing 06 and a rear bearing 07 are disposed between the outer shaft 04 and a hull 05, and a reversing front bearing 08 and a reversing rear bearing 09 are disposed between the inner shaft 02 and the outer shaft 04. Further, between the hull and the outer shaft and between the outer shaft and the inner shaft are disposed stern shaft seals 010 and 011, respectively, in a stern section.
The bearing disposed between the outer shaft 04 and the hull 05 is the ordinary type of stern tube bearing and has no peculiar technical difficulty. However, because the inner shaft 02 and the outer shaft 04 rotate in opposite directions, the bearing incorporated therebetween is confronted with some technical difficulties. The reasons are as follows.
FIG. 15 is a schematic cross-sectional view showing the mode of counter-rotation as viewed in the axial direction. Because the hull remains stationary, a normal fluid lubrication plain bearing is formed between the outer shaft 04 and the hull 05. However, because the inner shaft 02 and the outer shaft 04 are rotating in opposite directions as indicated by the arrows, fluid lubrication can hardly be achieved therebetween. Further, when the above two are rotating in opposite directions substantially at the same velocity, fluid lubrication (separation of the shafts by means of oil film) cannot be effected.
To cope with the aforementioned difficulty, such bearing systems as shown in FIGS. 16 and 17 were proposed in the prior art. The system of FIG. 16 is characterized in that a floating bushing 012 is disposed between the inner shaft 02 and the outer shaft 04 and held substantially stationary to form fluid lubrication oil films between the inner shaft and the floating bushing and between the floating bush and the outer shaft. FIG. 17 shows an outer shaft 013 having taper/land portions on its inner surface, that is, section A indicates a taper portion, B a land portion parallel to the surface of the inner shaft, and C a dynamic pressure generating taper portion effective when the whole system reverses (goes astern).
The conventional bearing system creates easily the fluid lubrication oil film (the dynamic pressure). However, if the marine vessel is running at a low speed, which arises during the service of the vessel, the oil film becomes thin as a result, metal-to-metal contact tends to occur on the bearing surface, and a rear end portion of the bearing 09 tends to cause offset contact as shown in FIG. 18 because of the propeller being overhung; thus, there is the problem that the bearing will seize.